Fair Value Measurements - Assets and Liabilities (Details) - USD ($) $ in Millions	Dec. 31, 2019	Dec. 31, 2018
Assets and liabilities measured at estimated fair value on a recurring basis
Debt	$ 628.8
Assets:
Gross Derivative Assets	261.1	$ 845.8
Liabilities:
Gross Derivative Liabilities	266.6	769.1
Fair value measured on recurring basis
Assets:
Total assets	270.4	852.1
Liabilities:
Total liabilities at fair value	266.6	769.1
Fair value measured on recurring basis | Level 1
Assets:
Total assets	148.3	585.6
Liabilities:
Total liabilities at fair value	177.6	556.5
Fair value measured on recurring basis | Level 2
Assets and liabilities measured at estimated fair value on a recurring basis
Debt	600.0	700.0
Notes receivable	21.0	28.2
Assets:
Total assets	111.4	265.6
Liabilities:
Total liabilities at fair value	80.9	212.4
Fair value measured on recurring basis | Level 3
Assets:
Total assets	10.7	0.9
Liabilities:
Total liabilities at fair value	8.1	0.2
Fair value measured on recurring basis | Commodity contracts
Assets:
Gross Derivative Assets	259.0	841.2
Liabilities:
Gross Derivative Liabilities	255.0	767.9
Fair value measured on recurring basis | Commodity contracts | Level 1
Assets:
Gross Derivative Assets	148.3	585.6
Liabilities:
Gross Derivative Liabilities	177.6	556.5
Fair value measured on recurring basis | Commodity contracts | Level 2
Assets:
Gross Derivative Assets	100.0	254.6
Liabilities:
Gross Derivative Liabilities	69.3	211.2
Fair value measured on recurring basis | Commodity contracts | Level 3
Assets:
Gross Derivative Assets	10.7	0.9
Liabilities:
Gross Derivative Liabilities	8.1	0.2
Fair value measured on recurring basis | Foreign currency contracts
Assets:
Gross Derivative Assets	2.0	4.6
Liabilities:
Gross Derivative Liabilities	11.6	1.2
Fair value measured on recurring basis | Foreign currency contracts | Level 1
Assets:
Gross Derivative Assets	0.0	0.0
Liabilities:
Gross Derivative Liabilities	0.0	0.0
Fair value measured on recurring basis | Foreign currency contracts | Level 2
Assets:
Gross Derivative Assets	2.0	4.6
Liabilities:
Gross Derivative Liabilities	11.6	1.2
Fair value measured on recurring basis | Foreign currency contracts | Level 3
Assets:
Gross Derivative Assets	0.0	0.0
Liabilities:
Gross Derivative Liabilities	0.0	0.0
Fair value measured on recurring basis | Cash surrender value of life insurance
Assets:
Cash surrender value of life insurance	9.4	6.3
Fair value measured on recurring basis | Cash surrender value of life insurance | Level 1
Assets:
Cash surrender value of life insurance	0.0	0.0
Fair value measured on recurring basis | Cash surrender value of life insurance | Level 2
Assets:
Cash surrender value of life insurance	9.4	6.3
Fair value measured on recurring basis | Cash surrender value of life insurance | Level 3
Assets:
Cash surrender value of life insurance	$ 0.0	$ 0.0